1. Field of Invention
The present invention relates to high electron mobility transistors (HEMT) and more particularly to transparent-gate HEMT employing indium tin oxide which can make HEMT more sensitive to the light wave.
2. Description of Related Art
In recent years, with the increasing of the mobile communication demand, high frequency devices such as high electron mobility transistor (HEMT), and heterojunction bipolar transistor (HBT) have a significant development. With respect to HEMT, pseudomorphic HEMT (PHEMT) and lattice match HEMT (LMHEMT) have better performance on current gain cut-off frequency (fT) and maximum oscillation of frequency (fmax) Because InGaAs in the InP substrate has higher electron mobility and higher peak electron velocity, LMHEMT shows better high frequency performance than PHEMT. Moreover, because the InP substrate is very expensive and fragile, the fabrication of the monolithic microwave integrated circuit (MMIC) is difficult.
In general, to use materials of different lattice constants is to place a buffer layer between them. This is done in the MHEMT or metamorphic HEMT, an advancement of the PHEMT developed in recent years. In the buffer layer is made of AlInAs, with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the GaInAs channel. This brings the advantage that practically any Indium concentration in the channel can be realized, so the devices can be optimized for different applications (low indium concentration provides low noise; high indium concentration gives high gain). The MHEMT device can have high frequency performance close to InP LMHEMT. The MHEMT device can save fabrication cost and make fabrication process easier.
Moreover, the HEMT is also called a heterostructure field effect transistor (HFET). The HEMT is a field effect transistor (FET) incorporating a junction (i.e. a heterojunction) between two materials with different band gaps as the channel instead of a doped region. The heterojunction created by different band-gap materials forms a quantum well in the conduction band on the GaAs side where the electrons can move quickly without colliding with any impurities because the GaAs layer is undoped, and from which they cannot escape. The effect of this is to create a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel high electron mobility. This layer is called a two-dimensional electron gas.
Referring to FIG. 7, the flowchart comprises an optical signal 1, a photodiode 2, a mixer 3, a local oscillation signal 4, a band pass filter 5, an amplifier transmission link 6, and a radio frequency signal 7. HEMT is a useful device to combine fiber system and radio communication system. When the HEMT is illuminated by light, the channel layer can absorb the optical signal. There are two types of the photoresponse. One type is referred to photovoltaic effect, whereas the other type is referred to photoconduction effect. The modulating optical signal illuminates into the device, the device can mix the local oscillation signal 4 and the modulating optical signal. Meanwhile, we can integrate the detector and mixer into one device to simplify the whole design. Then, we can also integrate far end microwave transmission system into a chip to lower the cost of the optical-microwave network, and indirectly make fiber to the home (FTTH) and fiber to the building (FTTB) become possible.
Referring to FIG. 8, the conventional gate metal is Ti/Au (20 nm/190 nm). The conventional gate metal is illuminated by −9 dBm to 0 dBm, 1.55 μm single mode laser. The bias point, Vd, equals to 0.45V at the most sensitive position to the light wave. The conventional gate metal exists high difficulty in mixing technology and has poor mixing efficiency and high cost. Thus, the need for improvement still exists.